Geology Adventures Essay


            This is the beginning of the tour.  We are standing at the waterfront, with downtown Hilo behind us and looking out over Hilo Bay.  Hilo Bay is well known for the tsunamis that have come ashore here.  The tsunamis that visit here are usually the result of Pacific Rim earthquakes rather than local tremblers.   The geography of the bay adds to the strength of the tsunami.  The waves build quickly due to the steep slope offshore and they lose little energy.  Once in the bay they reflect from one side to the other, and when the crests of the reflected waves collide, they build one very high wave (Hazlett 1996, p. 62).

            This bay is also where the lava flows from Mauna Loa meet Mauna Kea.  Part of the city is on a lava flow from the 1881 eruption of Mauna Loa.  The rest of the city is on soil that was created by the weathering of lava flows and ash falls from between 14,000 and 24,000 years ago.  Just south of the city, right by the zoo, are lava flows that are only 1,500 years old (Hazlett 1996, p. 57).

            Mauna Loa is one of the most active volcanoes on Earth.  It has erupted 15 times since 1900.  Its 1984 eruption sent lava to within four miles of Hilo (University of North Dakota 2003, p. 1).  Caution must always be exercised when deciding land use issues around active volcanoes, and Hilo’s proximity to Mauna Loa makes that even more important.

            Hilo has all of the amenities you would expect to find in a moderate sized city.  It is the islands largest city with a population of about 40,000.  There are abundant snorkeling opportunities, and the Pacific Tsunami Museum offers educational programs about this frequent visitor here. Hilo can be used as the gate way to the Hawaii Volcanoes National Park.  And we will be doing just that.  Our trip will take us up Highway 11 to the Kilauea Caldera.  From there we will drive down the Chain of Craters road to visit the Holei Sea Arch.  We will continue back to Hilo and visit the “Boiling Pots” series of plunge pools.  After that, we will terminate our trip at the Kaumana Cave, which is in reality a skylight lava tube.


            Here we are just inside the Hawaii Volcanoes National Park at the Volcano House hotel.  The park was established in 1916 and showcases over 70 million years of volcanic activity.  The Volcano House is Hawaii’s oldest operating hotel, opening in 1846 (US Park Info 2006).  From the porch of the Volcano House you can see all of the Kilauea Caldera (Hazlett 1996, p. 65).

            The caldera is about 2 miles wide and over 3 miles long.  It is also about 450 feet deep.  A broad lava shield named Halema’uma’u is on the floor of the caldera.  The caldera is slowing being filled from lava overflowing from the shield and surrounding fissures.  In a few centuries the entire caldera may be filled with this lava (Hazlett 1996, p. 65).

            The caldera was formed during or immediately after the large eruption that took place in 1790.  In 1823 the depth of the caldera was reported to be over three times deeper than it is today (Hazlett 1996, p. 65).  The caldera floor has collapsed several times since its formation, just to be filled in by lava again.  It is one of the most active volcanoes, and over 90% of the surface of the caldera is less than one thousand years old (Yoshioka 2002).  Because of the constant activity here, visitors must be cautious at all times.  Two Hawaiian chiefs would have been better off had they heeded this advise.

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During the eruption of 1790 these two Hawaiian chiefs, Kamehameha and Keoua,  were at war against each other.  Soldiers of  Keoua reached the summit and made camp.

That night the eruption took place, and they refused to go on.  The eruption continued for two days, in spite of the offerings they made to the volcano god Pele.  They broke camp on the morning of the third day, but had not advanced far before an earthquake and large eruption inundated them with ash, cinders, and hot gasses.  The survivors found the others much as the bodies were found at Pompeii, lifelike, preserved in their last moments (Hazlett 1996, p. 66-67).

Stop #3:  Holei Sea Arc.

            We have traveled from the caldera down the Chain of Craters road to the coast.  We are standing above the Holei sea arch.  Sea arches are formed by differential erosion caused by wave action on lava cliffs.  These cliffs are common in Hawaii.  As the waves erode at sea level, they undercut the cliffs.  Since the densities of the lava flows are not uniform, the erosion is irregular.  This differential erosion forms the sea arches.  Eventually, the Holei Sea Arch will collapse, forming a Sea Stack (Decker 1992, p. 21).

That is the scientific version of how sea arches are created.  Hawaiian legend states that they were the houses of Pele.  Pele and her sister Namakaokahai, the goddess of the sea, would have tremendous fights.  Pele would build her home, just to have her sister destroy it.  Then she would build another one, with the same end result (Decker 1992, p. 21).

Stop #4:  The Boiling Pots.

            We have traveled back to Hilo and up Pe’epe’e Falls Street to the Boiling Pots.  These are a series of plunge pools formed as the Wailuku River flows through lava filled gorges.  The river

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is cutting its channel along a previous channel.  Some of the lava flows here are about 3500 years old, while others are over 10,000 years old.  The Boiling Pots are cut from the younger lava flows (Hazlett 1996, p. 58-59).

            The Pots are formed from the differential erosion from the rain runoff that forms the river.  The river flows between the shields of the Mauna Loa and Mauna Kea volcanoes.  This forms a natural channel for the river to follow (Albion College 2004).  Along the gorge you can also see columnar basalt.  They formed as the basalt cooled from the bottom to the top, forming a hexagonal column.  These columns are about 40 cm across and are 10 to 12 meters tall (Albion College 2004).

            These features, like all features in a volcanically active region, will last only until the next lava flow destroys them.  Even though the pots may look inviting, thee are whirlpools that form as the water flows from one pot to another.  Therefore they are not safe to enter (Hazlett 1996, p. 59).

Stop #5:  Kaumana Cave.

            This brings us to our last stop and the end of the tour.  We are at the Kaumana cave, which is in reality a skylight a 25 mile long lava tube from the 1881 eruption of Mauna Loa (Hawaiian Style Travel 2006).  Lava tubes are formed as molten lava differentially cools.  The surface layers of the flow cool first, forming a crust over the still molten lava.  Since the crust is a good insulator, the lava continues to flow (Show Caves 2005).

            Once the lava flow stops, the remaining molten material in the tube continues to drain out, leaving an opening at the end.  This causes the cave formation we see here.  The top is eroded so this is a skylight, but the overgrowth hides the light (Show Caves 2005).  It is said that

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in 1881 Princess Ruth camped in front of this lava flow and prayed to Pele in order to save Hilo.  The flow stopped within a mile and a half of the city, sparing it (Hawaiian Style Travel 2006).

            On this trip we have seen several geologic formations.  They all have volcanic origins in common, but they are all unique.  The forces of water erosion on lava-formed structures created the sea arch and the Boiling Pots.  Volcanic activity directly formed the caldera and this lava tube.  This same volcanic activity has threatened Hilo and will undoubtedly threaten the city again.  We will never be able to stop flowing lava, but by observing the volcanoes, we may someday be able to predict the eruptions with some degree of accuracy.  Thank you for your attention and I hope you all have learned a little about volcanism and its forces first hand.


Albion College. (2004). Forming of the Boiling Pots. Retrieved April 18, 2007, from Albion

   College Web site:

Decker, R., & Decker, B. (1992). Roadside Guide to Hawaii Volcanoes National Park.

   Retrieved April 18, 2007 from Dartmouth College  Web site:

Hawaiian Style Travel. (2006).  Kaumana Cave. Retrieved April 18, 2007 from Hawaii Style

   Travel  Web site:

Hazlett, R., & Hyndman, D. (1996). Roadside Geology of Hawai’i.  Missoula, MT: Mountain


Show Caves. (2005). Classification of Caves: Lava Tubes. Retrieved April 18, 2007 from Show

    Caves  Web site:

University of North Dakota. (2003). Mauna Loa. Retrieved April 18, 2007 from the University

    of North Dakota  Web site:

Yoshioka, M. (2002). Kilauea Caldera.  Retrieved April 18, 2007 from the La Piertra High

    School  Web site: